1. Field of the Invention
The present invention relates to AC motors. More particularly, the present invention relates to an apparatus to be used with a pulse width modulated (PWM) inverter driving an AC motor to eliminate unintended voltage pulses caused by discontinuities in a modulating command signal.
2. Description of the Art
Many motor applications require that the motor be driven at various speeds. Motor speed can be adjusted with an adjustable speed drive (ASD) and which is placed between a voltage source and an associated motor that can excite the motor at various frequencies.
One commonly used type of ASD uses a pulse width modulated (PWM) inverter and associated PWM controller which can control both voltage and frequency of signals that eventually reach the stator windings of a motor. Referring to FIGS. 2(a) and 2(b), a PWM controller receives both a modulating command signal 16 and a carrier signal 18, compares the command and carrier signals 16, 18 and produces a firing signal 20. When the command signal 16 is greater than the carrier signal 18, the firing signal 20 is high. When the command signal 16 is less than the carrier signal 18, the firing signal 20 is low.
The firing signal 20 is used to control an associated PWM inverter. The inverter produces a series of high frequency voltage pulses that excite the stator windings of a motor. Referring also to FIG. 2(c), an exemplary sequence of high frequency pulses 40 that an inverter might provide to a motor can be observed along with an exemplary low frequency alternating voltage 42 and related alternating current 44. The high frequency pulses 40 are positive when the firing signal 20 is high and negative when the firing signal 20 is low.
By varying the widths of the positive portions 43 of each high frequency pulse relative to the widths of the negative portions 45 over a series of high frequency pulses 40, a changing average voltage 42 can be generated. To produce a sinusoidal average voltage 42, a simple sinusoidal modulating command signal 16 can be used.
The changing average voltage 42 defines the low frequency alternating voltage that drives the motor. The low frequency alternating voltage 42 in turn produces a low frequency alternating current 44 that lags the voltage by a phase angle .phi.. As well known in the art, the motor operates at the frequency of the alternating current 44.
By changing the frequency of the sinusoidal command signal 16, the frequency of the alternating current 44, and thus the motor speed, can be altered. For example, by increasing the frequency of the command signal 16, the frequency of the alternating current can be increased and motor speed can in turn be increased. Motor speed can be decreased by decreasing the sinusoidal command signal 16 frequency. In addition, by changing the peak-to-peak amplitude variation of the command signal 16 while maintaining a constant frequency, the amplitude of the stator winding current can be altered.
In theory, a PWM inverter can be used to drive a motor accurately at various motor speeds. In reality, however, due to controller-inverter system noise and unforseen command signal discontinuities, often the alternating voltage commnad produces excessive switching of the power devices.
To correct for errors in motor current, many PWM controllers include a feedback loop which compares actual motor current to desired motor current and increases or decreases the command signal 16 in order to compensate for current errors. While a properly designed feedback loop can correct for noise and signal discontinuities, often an abruptly corrected command signal can introduce greater error than it eliminates.
Referring to FIGS. 3(a) and 3(b), a corrected command signal 16' and a carrier signal 18' along with a resulting firing signal 20' can be observed. Prior to t.sub.1, the command signal 16' is greater than the carrier signal 18' and the firing signal 20' is appropriately high. At t.sub.1, the command signal 16' crosses the carrier signal 18' and the firing signal 20' goes appropriately low.
At t.sub.2, the amplitude of the command signal 16' is increased 46 to correct for a deviation between the desired and actual stator winding currents. When corrected, the command signal 16' becomes greater than the carrier signal 18' and the firing pulse 20' goes high. The firing pulse 20' remains high until the command signal 16' again crosses the carrier signal 18' at t.sub.3. At t.sub.4 the command signal 16' again goes above the carrier signal and the firing signal 20' goes high.
The command signal correction 46 at t.sub.2 results in a command signal discontinuity at t.sub.2 and a "double crossing" between the carrier and command signals 18', 16' first at t.sub.1 and then at t.sub.3. The double crossing in turn produces an additional and unintended firing pulse 22. Referring also to FIG. 3(c), the unintended firing pulse creates an unintended additional high frequency pulse 47 which ultimately increases the switching losses
Beside feedback, double crossings can be caused by command signal discontinuities and generally by system noise. Together, noise, signal discontinuities, and feedback corrections can produce enough double crossings to produce imprecise motor operation to a degree which is intolerable for many motor applications.
One way to eliminate the effects of double crossings is to employ a low pass filter just prior to the comparator circuit. The low pass filter allows the feed back loop to correct the command signal but slopes the correction so that it takes place more gradually (i.e. the correction is sloped so as to be less steep than the carrier signal).
Unfortunately, low pass filter operation is sensitive to motor application, motor size, and the carrier frequency. Motor application and size affect the amount of noise in a system and thus can instantaneously affect the difference between carrier signal slope and the slope of a corrected command signal. Similarly, carrier frequency clearly affects the difference between carrier signal slope and the slope of a corrected command signal. Thus, a low pass filter designed to operate under one set of conditions cannot operate effectively in all applications to eliminate the effect of double crossing signals.
Thus, it would be advantageous to have a method or apparatus which could eliminate the effects of double crossing. Ideally, the method or apparatus should be independent of motor application and size and independent of carrier signal frequency.